Balance detector used in electronic analogue-to-digital converter



Jan. 27, 1959 M. L. KUDER 2,871,351

BALANCEDETECTOR USED IN ELECTRONIC ANALOGUE-TO-DIGITAL CONVERTEROriginal Filed Jan. 9. 1953 2 Sheets-Sheet 1 f2 PROGRAMMING 'f Z EOLSCILLA TOE 5W1 TCH RESET, N JTOP f 3 1 5 4 i jg A5752 BALANCE BLANKINGE ogmmm J: EK nerecroz DECADE "N175 2; couuree luv/c4102 E 5 v I f i 2DECADE 10's 5 COUNTER mo/cno/e Q DECADE I003 ecu/wee INDICATOR SOURCE15-6 R 41 Y 37 13.70 7' 5/24 ,wva 42 07CTOR a v f 222 2 m AMPLIFIERINVENTOR F 4 Ml'lfonLKuder BY M '1? 1 AGENT INTEGRATOR 6 Jan. 27, 1959ub 7 2,871,351

BALANCE DETECTOR USED IN ELECTRONIC ANALOGUE-TO-DIGITAL CONVERTEROriginal Filed Jan. 9. 1953 2 sheets sheet 2 II V v ll 17 v REGULATE-DPULSE CATHODE 7 souece 5 FOLLOWER I7 I6 [5' REGULATED SOURCE DUMP\SIGNAL INVENTOR OUTPUT T 5 Ml'lfon L.Kuder- BY M M AGENT BALANCEDETECTOR USED IN ELECTRONIC ANALOGUE-TO-DIGITAL CONVERTER Milton L.Kuder, Washington, D. C., assignor to the United States of America asrepresented by the Secretary of Commerce Original application January 9,1953, Serial No. 330,599, now Patent No. 2,761,968, dated September 4,1956. Divided and this application December 15, 1955, Serial No. 553,398

Claims. Cl. 250-27 The present invention relates to an electronicconversion system and in particular to a system which converts ananalogue parameter into magnitude-related digits. This application is adivision of applicants copending application Serial No. 330,599 filed onJanuary 9, 1953, now U. S. Patent No. 2,761,968 granted on September 4,1956.

In certain applications, electronic digital indicating systems offerimportant advantages over more conventional analogue indicators-those inwhich a pointer moves over a continuous scale, or in which theindication otherwise varies continuously with the measured quantity. Inmoving-pointer meters, speed of response is usually limited bymechanical characteristics of the meter, and the accuracy to which themeter can be read-about 0.5 percent in precision analogue metersislimited by a number of factors. digital'computers in contrast withanalogue computers has clearly focused attention on the fact thatindicating and recording instruments are almost exclusively of theanalogue type, thereby making the indicating and computing instrumentsincompatible.

Another field in which digital systems have distinct advantages is thetelemetering field. The usual practice is to convert the informationunder investigation into electrical voltage levels and then send thisinformation to a cen.ral control point. is that random noise may producevariations in the voltage levels transmitted, thereby producingerroneous information at the other end of the system. in order toeliminate this difficulty it is proposed that the information betransmitted in the form of voltage pulses, the number of pulsestransmitted being indicative of the voltage level which represents theinformation desired.

The primary object of the invention is to provide an electronicinstrument which accurately converts an analogue parameter intomagnitude-related digits.

Another object of the invention is to 'provide an analogue-to-digitalconverter which samples the unknown parameter at least 100 times asecond.

Another object of the invention is to provide an analogue-to-digitalconverter employing novel integrator and balance-detecting means.

Another object of the invention is to provide an integrator forproducing a stair-step voltage in which each increment of voltage ismaintained constant over a very wide range.

Another object of the invention is to provide a first feedback circuitin the integrator to insure that the system will follow a linearfunction over a wide range.

Another object of the invcnticn is to provide a second feedback circuitin the intergrator in order to correct certain effects not corrected bythe first feedback circuit.

Another object of the present invention is to provide a balance detectorwhich will determine to a high order of accuracy when the analoguevoltage and the output of the integrator are equal.

Also the increasing importance of t 1 The disadvantage in this system2,871,351 Patented Jan. 27, 1959 "ice Figure 2 is a simplified circuitdiagram of the integrator I showing the first feedback path.

Figure 3 is a complete circuit diagram of the integrator showing thefirst and second feedback paths.

Figure 4 is a circuit diagram of the balance detector.

In Figure 1 there is shown a programming oscillator 1, the'first outputof which feeds the electronic switch 2 and the second output of whichfeeds the blanking generator 3 and reset generator 4. The output of theelectronic switch is connected to the input of'the master oscillator 5,the first output of which is connected to the in tegrator 6 and thesecond output of which is connected to the decade counter 7. The outputof the integrator 6 is fed to a first input of the balance detector 8,the second input of which is connected to the analogue quantity to bemeasured. The output of the balance detector is connected to the secondinput of the electronic switch. The decade counter 7 is connected to asecond decade counter 9 which is connected to a third decade counter 11.The outputs of decade counters 7, 9, and 11 are connected to the unitsindicator 12, tens indicator 13 and hundreds indicator 14, respectively.The output of the blanking generator 3 is connected tothe indicators 12,13, and 14, and the output of the reset generator 4 is connected to thedecade counters 7, 9, and 11.

The programming oscillator 1 puts out a square wave, the positive halfof which causes the blanking generator to send out a pulse to theindicators 12, 13, and 14 to prevent them from registering during acount. The same pulse causes the reset generator 4 to reset the decadecounters 7, 9, and 11 to zero count. The other output of the programmingoscillator 1 causes the electronic switch to turn on the masteroscillator 5. Sufficient time delay must be provided so that the masteroscillator is not started until the indicators have been blanked and thecounters have been reset to zero. The'first output of the masteroscillator 5 is fed to the integrator 6, each pulse of the oscillatorcausing the output of the integrator to be increased by a single voltageincrement of a precisely determined value. For each pulse sent by themaster oscillator to the integrator 6, there is a corresponding pulsesent to the decade counter 7, which there fore counts the number ofpulses sent to the integrator; When the decade counter 7 has counted to10, it sends a pulse to the decade counter 9, which in turn sends apulse to decade counter 11 when it has made a count of 10. Therefore thedecade counter 7 is a units counter, decade counter 9 is a tens counter,and decade counter 11 is a hundreds counter. if necessary, more counterscan be added to increase the count over the 999 available in the systemdescribed. Each time the decade counter 7 receives a pulse from themaster oscillator, it puts out'a pulse to the unit indicator, causing itto increase its count by one. The tens indicator l3 and the hundredsindicator 14 receive pulses in the same manner from the decade counters9 and 11, respectively. Therefore when the master oscillator has beenstopped in the manner to be explained later, and the indicators areunblanked, they will indicate the number of pulses put'out by the masteroscillator during the integrating interval.

The output of the integrator 6 is increased by a predetermined valueeach time the integrator receives a pulse from the master oscillator.The incremental voltage increase at the output of the integrator iscontrollable. The specific increment employed depends upon the range ofvoltages to be measured and the degree of accuracy required.

When the voltage level of the output of the integrator is equal to thevoltage level of the unknown voltage E the balance detector sends apulse to the electronic switch, causing the switch to send a pulse tothe master oscillator, preventing it from operating further. Since thedecade counters have counted the number of pulses necessary to producean integrator output whichis exactly equal to the unknown voltage, andsince the increase in the output of the integrator for each pulse fromthe master oscillator is known, it is a simple matter to convert thereading on the indicators 12, 13, and 14 to the voltage E The frequencyof the programming oscillator must be such that the master oscillatormay complete its maximum number of oscillations before the programmingoscillator has completed a half cycle. That is, the master oscillatormust be able to complete 999 cycles, the maximum count possible with thenumber of decade counters shown, before the programming oscillator sendsa control pulse to the blanking and reset generators. The negativeoutput from the programming oscillator has no effect upon the electronicswitch or the reset mechanism, but it causes the unblanking generator toproduce an output which unblanks the indicators 12, 13, and 14 andallows them to be read or to produce a permanent record, depending uponthe type of indicators used. At the beginning of the next cycle of theprogramming oscillator, the indicators are again blanked and the resetgenerator 4 sends out a pulse which resets the decade counters, which inturn reset the indicators.

The equipment may all be at one location or the decade counters andindicators may be located at some distance from the rest of the unit. Inthe latter case contact between the two locations may be established bywire orradio.

As previously pointed out, the integrator 6 is the element which governsthe overall accuracy of the system. This unit must put out a preciseincrease in voltage for each pulse received from the master oscillator.In order to accomplish this result the analogue integrator operateson'the basis of a constant coulomb capacitor counter. The method ofcounting is essentially accomplished by transferring a fixed charge ofelectricity into a large capacitor from a small capacitor which haspreviously received a unit charge. In order that the system may follow alinear function over a wide range, it is necessary that the unit chargetransfer from the smaller capacitor to the larger capacitor shall beaccomplished by a complete transfer of the charge at each increment.Moreover, the unit charges placed into the smaller capacitor must bemaintained constant for each increment over the entire range.

These results are obtained in general by the simplified circuit of theintegrator shown in Figure 2. In this figure a regulated pulse source 16puts out a precisely determined pulse amplitude each time it receives anoutput from the master oscillator over the wire 15. The pulse is fed toa series circuit consisting of the small capacitor 17, diode 18, and alarge capacitor 19. Although the amplitude of the pulses must beprecisely determined, the pulse width need not be because the values ofcapacitors 17 and 19 are chosen so as to present a low impedance circuitto the output and therefore the capacitors become almost completelycharged long before the pulse is completed. The input to a cathodefollower circuit is connected between the diode 18 and capacitor 19, andthe output of the cathode follower is fed back to the junction ofcapacitor 17 and diode'18 through the diode 4 21. The output is takenbetween the diode 18 and capacitor 19. Each voltage pulse put out by theregulated source 16 is divided between capacitors 17 and 19 according tothe formula where 13,; is the amplitude of the pulse put out by thesource 16. It will be apparent that capacitor 17 is repetitively chargedand discharged by the regulated pulse source 16 in a manner such thatthe unit charge in 17 is transferred into the larger capacitor C-7through transfer diode 18. During each negative transition of theregulated pulse source the potential across the capacitor 17 is restoredby the feedback cathode follower and the diode 21 to the potential whichhas just previously been accrued on the capacitor 19. This feedbackresults in the capacitor 17 passing through the same change of chargefor each pulse from the regulated pulse source. If the capacitor 17passes through the same change of charge on each positive transitionfrom the pulse source, then the same number of coulombs will betransferred into capacitor 19 upon each pulse or increment. This resultsin a linear summation of the constant voltage E The manner in which thisconstantcoulomb charge on the capacitor 17 is obtained can bedemonstrated by an analysis of the operation of the system. The diode 18is inserted in series with the capacitors 17 and 19 so as to break thispath during the negative transition of the pulse source and therebyprevents discharge of the capacitor 19 during this period. When apositive pulse is applied to the input of this circuit the plate of thediode 18 is driven positive with respect to the cathode and the chargesupplied by the pulse source divides between the capacitors 17 and 19.Assuming that a -volt pulse is applied and that the capacitor 19 is 99times larger than capacitor 17, then one volt will appear across thecapacitor 19. The right-hand plate of capacitor 17 will be at a one-voltlevel owing to the accrued potential across the capacitor 19. If nofeedback were provided, during the negative excursion of the pulse fromthe source 16, the capacitor 17 would stay fully charged, and theright-hand plate of the capacitor would be at a 99 volts when theleft-hand plate was returned to zero volts. As a result no current wouldbe passed by this circuit during the next pulse, since there would be aback voltage of 100 volts across the diode 18. In order to eliminatethis effect the right-hand plate of the capacitor 17 is restored duringeach negative excursion of pulse 16 to the same potential as the upperplate of the capacitor 19 by means of the feedback path consisting ofthe cathode follower 20 and diode 21. During the negative transition ofthe pulse, the cathode of diode 21 becomes slightly negative withrespect to its plate and allows conduction through this feedback path.As a result the cathode and plate of the diode 18 will be at the samepotential after this restoration and there will be no back voltageacross the diode 18 prior to the application of the next positive pulse.This arrangement causes the IOU-volt pulse source always to drive thecapacitor 17 through the same change of charge, and since the value ofthe capacitor 17 remains constant, it will always accrue the samecoulomb charge. Therefore the feedback path allows this system tooperate on a constant-coulombcapacitor counter basis.

Since the cathtode follower 20, shown in Figure 2, does not provideunity gain, it follows that the plate and cathode of the diode 18 wouldnot be restored to exactly the same potential. This difficulty is takencare of by a second feedback path as shown in the circuit of Figure 3.In this circuit the components which correspond to the components inFig. 2 carry the same reference numerals. The junction of the diode 18and capacitor 19 is connected to the grid 22 of the cathode follower 20.The output of the cathode follower is connected through the triode 27which receives the dump signal to discharge.

the capacitor 19 after a count has been completed. A portion of thecathode follower output which is determined by the relative valuesofresistors 28 and 30 is fed back to the pulse'source 16 over the lead 29which defines a second feedback path. The output of the system is takenbetween diodes 21 and 24. It was found necessary to insert the capacitor23 in the first feedback path to compensate for the difference ofpotential between the grid and cathode of the cathode follower. That is,since all cathode followers have some stagger in the voltage between thegrid control and cathode output it is necessary to subtract suchdifference by A. C. coupling through the capacitor 23. Theintroductionof capacitor 23 also causes some degeneration in the system,since on the negative transition of the pulse source 16, the capacitors23 and 17 and diode 21 constitute the same type ofcircuit as capacitors17 and 19 and diode'18. By properly choosing the values of thecapacitors 17, 19, and 23, this effect can be maintained at a very lowvalue. That is, if the capacitor 23, is, for instance, 99 times largerthan capacitor 19, then for each one-volt positive accrual on capacitor19, there will be a 0.01 volt negative accrual in capacitor 23. Thisdegeneration would introduce a nonlinearity of only one percent.However, this nonlinearity and the nonlinearity introduced by the factthat the cathode follower falls at little short of providing an idealunity gain canbe corrected by the use of the second feedback path. Ifthe degenerative signal voltage which accrues in the capacitor 23 isequal to one percent of the output voltage and the degenerative gaincharacteristics of the cathode follower introduce another two percenterror in the system, values of resistors 28 and 30 are so chosen that avoltage equal to 3 percent of the voltage output is fed back to and inseries with the prime voltage reference source. This feedback voltageincreases the output of the source by the total degenerative percentage.

In other words, assuming an initial voltage pulse output of 100 volts,and therefore a one-volt accrual across the capacitor 19, approximately0.03 volt will be fed back to the regulated source and the next pulsefrom that source will be equal to 100.03 volts rather than 100 volts.Therefore, the degenerative errors will be corrected during the next andsucceeding pulses.

At the end of a count a dump signal is sent to the triode 27 from theelectronic switch and the capacitor 19 is discharged through the triode.This returns the cathode of the cathode follower to its initial minimumpotential, and the capacitor 23 is returned to its initial charge bydischarging the small negative voltage accumulated on its left platethrough the diode 24 to ground. At the same time the capacitor 17 isdischarged through the diode 18 and triode 27, thereby restoring thesystem to its starting condition. The resistor 26 stops drift duringstatic times; that is, this resistor in conjunction with diode 24prevents the capacitor 23 from accruing a charge during static periods.

The output of the integrator 6 (Fig. 1) is fed to the balance detector 8wherethe integrated voltage EE from the integratoris compared with theanalogue E Referring to Figure 4, the voltage 2B; is applied acrosscapacitor 31 in series with the capacitor 32 across which is applied ananalogue voltage 15,. These capacitors are connected in series with theprimary 33 0f the transformer 34, which is shunted by capacitor 35, andwith the fullwave rectifier 36. The primary 33 and capacitor 35 andsecondary 37 and capacitor 45 are tuned to the secondharmonic of thefrequency of the voltage from the source 40. The rectifier circuit 36consists of the secondary 37a of transformer 38 in series with thediodes 39 and 41, which have their cathodes connected together. Thejunction of these two cathodes is connected to the primary 33 oftransformer 34. The R.:F. source 40 is coupled into the circuit throughthe transformer 38. The voltage E is applied to the circuit so that apositive voltage appears at the right side of the capacitor 32. Thevoltage 2E is applied so as to buck this voltage; that is, the positiveside of the capacitor 31 is on the left side. Initially the voltage E islarger than 2E and therefore the voltage appearing at the cathodes ofthe diodes 39 and 41 is positive with respect to the voltage appearingat the plates of these diodes. As a result there can be no conductionthrough the diodes and the R.-F. voltage coupled in the rectifiercircuit is blocked. The full-wave rectifier circuit 36 can beconsidered, for purposes of explanation, as a gating device whichnormally isolates R.-F source 40 from the tuned circuits 34, 35, 45. Aslong as the analogue voltage E is larger than the 2B,, voltage, theresulting difference voltage biases the diodes 39 and 41 of rectifier 36to cut-off and the rectifier will not conduct signals from R.-F.'source40. The ZE voltage has a stepped waveform and the peak voltage generatedby the R.-F. source 40 isslightly less than an increment of thestep-voltages EE When the cumulative amplitudes of the EB step-voltagesare within a fraction of one such increment or less, the combination ofsuch voltage amplitudes with the voltage ampli-v tude generated by R.-F.source 40'will render the diodes 39 and 41 conductive and detector 36will therefore be energized to establish an operative connection betweensource 40 and the tuned circuit comprising transformer 34.

The full-wave rectifier 36 also inherently serves as a second harmonicgenerator with respect to the signals from R.-F. source 40 and since thecircuit comprising transformer 34 is tuned to such second harmonic, itwill be apparent that under the stated conditions of operation in whichEE approximately equals E a second harmonic signal will be fed to theL-F. amplifier 42. After amplification the signal is fed to the seconddetector 43 where it is converted to a D.-C. voltage and differentiatedso as to produce a stop-pulse signal which is sent to the electronicswitch 2.

It is apparent that the only time current can flow from the source ofunknown voltage E is at the one instant before shutofi when the voltages2E, and E are equal,

for immediately subsequent to the action of the stoppulse produced bythe second detector, ZE is returned to zero by the dump signal.Therefore the effective impedance of thi circuit to E is very high andpresents a negligible drain upon this source. Another importantadvantage of this system stems from the use of the two diodes to chopthe difference of EB, and E at a frequency that is the second harmonicof the oscillator frequency. If the two diodes do not have matchedcapacitances, which is usually the case, a voltage, which is in no wayrelated to the relative values of EE and E appears across the winding33. However, since this circuit is tuned to the second harmonic of thefrequency of source 40 this fundamental voltage has no effect. Also itwill be seen that the input signals may be chopped at any frequencydesired merely by changing theoscillator frequency and the resonantfrequency of the tuned circuits.

It will be apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementWithin the scope of my invention as defined in the appended claims.

7 What is claimed is: 1. In an apparatus ofthe type described, a balancedetector for generating a control signal upon consonance between theamplitudes of an applied analogue voltage E and a test voltage ZEcomprising means responsive to said E and 2E; voltages for producing avoltage corresponding to the dilference therebetween a second voltagesource having a predetermined frequency, circuit means responsive totwice the frequency of said second voltage source, and selectivelyenergizable means for operatively connecting said voltage source to saidresponsive means under a predetermined condition of operation in whichsaid E and EE; are substantially equal, said energizable meanscompriisng means biased by said difference voltage for normallydeenergizing said energizable means and for energizing said energizablemeans under said condition of operation in which said difference voltagehas diminished to substantially zero.

2. The invention of claim 1 in which said energizable means comprises arectifier for converting the output from said second voltage source intoa signal having twice the frequency of said source.

3. The invention of claim 1 in Which said circuit means comprises aresonance circuit tuned to the frequency of the output of said detector.

4. The invention of claim 3 including utilization means connected to theoutput of said tuned circuit means.

5. In an apparatus of the type described, a balance detector forgenerating a control signal upon consonance between the amplitude of anapplied analogue voltage E and the cumulative amplitudes of appliedincremental test voltages, comprising means responsive to said E andsaid incremental 2E voltages for producing a voltage corresponding tothe difference therebetween, a second voltage source of predeterminedfrequency having an amplitude slightly less than that of each of saidincremental test voltages, circuit means responsive to twice thefrequency of said second voltage source and s'electively energizablemeans for operatively connecting said voltage source-to said responsivemeans under a predetermined condition of operation in which said 2Evoltages is within a fraction of one increment or less of said Evoltage, said energizable means comprising means biased by saiddifierence voltage for normally deenergizing said energizable means andfor energizing said energizable means under said condition of operation.

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